III-V semiconductors are used to make photoelectric elements such as LEDs to emit light based on the electro-luminescence conversion effect. An LED is high in electroluminescence conversion efficiency but low in energy consumption. Hence, a lot of efforts have been made to develop LEDs for general lighting applications. There is a trend to use LEDs instead of current illuminative devices.
As disclosed in U.S. Pat. No. 6,765,237, a conventional LED includes a chip, a fluorescent layer provided on the chip and epoxy for packaging the chip and the yellow fluorescent layer. Based on the conversion effect, the chip emits blue light. The blue light turns the electrons of the fluorescent layer into an excited state from a ground state. In the excited state, the fluorescent layer emits yellow light. The blue light is mixed with the yellow light, thus providing WL. This is sometimes called “LED color-mixing technology.”
This conventional LED is the mainstream product since its making and using are simple. However, the fluorescent layer is vulnerable to heat generated from the chip so that the wavelength of the light emitted from the LED changes, and the intensity of the illumination or luminance of the LED decays. This is sometimes called “fluorescent decay.”
Currently, most LEDs emit WL based on the chemical color mixture. However, they suffer the above-discussed problems that have not been overcome. Therefore, such LEDs are not suitable for long-term applications.
Referring to FIG. 1, another conventional multi-chip LED lighting module includes a chip 1 for emitting red light (“RL”), another chip 2 for emitting green light (“GL”) and another chip 3 for emitting blue light (“BL”). The wavelengths and intensities of the light of the primary colors must be carefully selected to provide WL. Even with careful selection, WL only exists in an area where the light beams of the primary colors overlap. Light turns to the primary colors away from the area. There are various color blocks.
The illuminative angles of the chips can be enlarged to mitigate the effect of color blocks. However, human eyes are more sensitive to GL with a wavelength of 555 nm than any other light. This is called spectrum sensitivity as shown in FIG. 2. Moreover, the conventional LED shown in FIG. 1 causes glare to human eyes.
Referring to FIG. 3, another conventional lighting module includes chips 4, 5 and 6 for respectively emitting BL, GL and RL, a package 7 for wrapping the chips 4, 5 and 6, a substrate 8 for supporting the chips 4, 5 and 6 and the package 7 and scattering particles 9 provided in the package 7. The scattering particles 9 scatters and mixes the RL, GL and BL respectively emitted from the chips 4, 5 and 6 into WL.
The conventional lighting module shown in FIG. 3 provides good mixture of the RL, GL and BL. It has not been made available on the market because it exhibits unacceptable color blocks. Moreover, the chips 4, 5 and 6 are provided in the single package 7 so that this conventional lighting module suffers overheating and does not last long.
Another conventional lighting module includes a WL LED, an RL LED, a GL LED and a BL LED. The WL LED is used as a major lighting module, and the RL LED, GL LED and BL LED color temperature-compensating units. If necessary, at least some of the color temperature-compensating units are activated to emit light to compensate the changes in the color temperature of white light emitted from the WL LED due to the thermal drift of the wavelength. The brightness, color temperature and color blocks of this conventional lighting module change tremendously after the WL LED decays. Moreover, it is difficult and uneconomic to precisely control currents provided to the LEDs.
Moreover, the wavelength of light emitted from an LED is determined by the structure of the epitaxy, materials used therein and the matching of lattices. The wavelength of the light emitted from the LED suffers thermal drift. That is, at the moment when the multi-chip LED lighting is actuated, the intensity of the red light is high so that the white light tends to be a warm color. As the multi-chip LED lighting goes on, the intensity of the blue light gets higher so that the white light tends to be a cold color. The thermal drift of the white light might be too big to achieve a good white balance. The intensity of illumination would be compromised accordingly.
Therefore, the present invention is intended to obviate or at least alleviate the problems encountered in prior art.